For realization of an ultra high-speed optical transmission system of 100 Gbit/s or more, an optical multi-valued modulation method has come to the fore. In particular, a coherent receiving method such as DP-QPSK (Dual Polarization Quadrature Phase-Shift Keying) has attracted attention because of its advantages of enhanced optical noise immunity and compensation performance by electrical signal processing on wavelength dispersion distortion after photoelectric conversion. Application of the coherent receiving method to a transmission system has been actively discussed. An optical receiver used in the coherent receiver system comprises a local oscillation light generating apparatus for generating local oscillation light, a polarization beam splitter for separating a signal light and a local oscillation light into different output ports corresponding to a polarization state, an optical 90-degree hybrid circuit for wave-combining the signal light and the local oscillation light, a photoelectric conversion section for converting an output signal from the optical 90-degree hybrid circuit into an electrical signal, an AD converter for converting the electrical signal from the photoelectric conversion section into a digital signal, and a digital signal processing (DSP) circuit for processing the digital signal. By separately detecting an in-phase component and a quadrature component of interference light of the inputted signal light and the inputted local oscillation light, it is possible to obtain information of the inputted signal light.
Among construction parts of the optical receiver used in the coherent receiver system, as to the optical 90-degree hybrid circuit, a product constituted by a spatial optical system by combining bulk type optical parts has been already developed and provided on the market. On the other hand, a planar light wave circuit (PLC) constituted by optical waveguides produced on a planar substrate is superior to the above-described spatial optical system in terms of mass production capabilities and reliability. In addition, by adopting the PLC optical 90-degree hybrid circuit, for example, the feasibility in regard to integration of the polarization beam splitter and the photoelectric conversion section is increased as compared to the spatial optical system, enabling a provision of a smaller-sized optical receiver. Under these circumstances, it is expected to put the PLC optical 90-degree hybrid circuit into practice.
FIG. 1 is a construction diagram showing a conventional PLC optical 90-degree hybrid circuit. The conventional PLC optical 90-degree hybrid circuit is shown in PTL 1. PTL 1 relates to an optical delay interference circuit used for demodulation of a DQPSK (differential quadrature phase-shift keying) signal. This circuit itself does not correspond to the part constituting the optical receiver used in the coherent receiver system, but includes, as a part of the circuit, a function as the optical 90-degree hybrid circuit which combines two optical waves and separates the combined wave into an in-phase component and a quadrature component. Hereinafter, the in-phase component is referred to as “I component”, and the quadrature component is referred to as “Q component”. In FIG. 1, among the optical circuits described in PTL 1, the construction of a circuit part alone necessary for realizing the optical 90-degree hybrid circuit is extracted to be shown.
Here, an explanation will be made of a propagation process of light inputted into the conventional PLC optical 90-degree hybrid circuit shown in FIG. 1. A signal light inputted via an input waveguide 1a from the PLC external is branched into two lights by an optical splitter 2a. A local oscillation light inputted via an input waveguide 1b from the PLC external is branched into two lights by an optical splitter 2b. The lights branched into two portions by the optical splitter 2a are inputted into two optical couplers 3a and 3b via arm waveguides 10a and 10b. The lights branched into two portions by the optical splitter 2b are inputted into the two optical couplers 3a and 3b via arm waveguides 10c and 10d. The signal light and the local oscillation light inputted into each of the optical coupler 3a and the optical coupler 3b are combined to be interfered with each other, which is branched into two lights for output so that a phase difference between the interference lights becomes 180 degrees. The interference lights of the signal light and the local oscillation light outputted from the optical coupler 3a travel via output waveguides 4a and 5a and are outputted into a differential optical receiver section 6a formed as an external circuit and serving as a photoelectric conversion section. The interference lights of the signal light and the local oscillation light outputted from the optical coupler 3b travel via output waveguides 4b and 5b and are outputted into a differential optical receiver section 6b formed as an external circuit and serving as a photoelectric conversion section.
A 90-degree phase shift section is provided in any one of the four arm waveguides 10a, 10b, 10c, and 10d. Thereby the interference lights outputted via the output waveguides 4a, 4b, 5a and 5b from the respective optical coupler 3a and the optical coupler 3b can be differentially demodulated by the differential optical receivers 6a and 6b to separate I component and Q component of the inputted modulation signal. Here, for simultaneously detecting I component and Q component of the modulation signal, it is necessary that waveguide lengths of the two arm waveguides 10a and 10b for transmitting the signal lights branched in the optical splitter 2a each are made equal and waveguide lengths of the two arm waveguides 10c and 10d for transmitting the local oscillation lights branched in the optical splitter 2b each are made equal excluding the 90-degree phase shift section 7. Further, waveguide lengths of the four arm waveguides 10a, 10b, 10c and 10d each are made equal excluding the 90-degree phase shift section 7, and thereby, it is possible to use this circuit also as the optical 90-degree hybrid circuit constituting the optical delay interference circuit for receiving the differential phase modulation signal such as DQPSK.